FIG. 5 shows a hydraulic circuit, containing a four-position selector valve 101, which has been used with a conventional construction machine, a material handling machine or the like, and which is adapted to be operated by a manual control lever 102. The four positions of the four-position selector valve 101 are the "up" position, the "hold" position, the "down" position, and the "floating" position. For example, the selector valve 101 can be set at the "floating" position so as to cause the blade cylinders 103 to be freely operatable while the vehicle body is moved backwardly in order to level the land with the use of a blade (not shown), under the deadweight of the blade. Further, in order to enhance the manipulability of the control lever 102, a detent 104 is incorporated so as to hold the "floating" position when the control lever 102 is moved to the "floating" position and then released by the operator.
In recent years, a closed-center load sensing circuit has been used in order to satisfactorily control the vertical position and the operational speed of the blade. In the hydraulic circuit shown in FIG. 6, each of selector valves 111, 112 has three positions and is changed over by a pressure proportional control valve 113, 114. However, no "floating" position is provided in the selector valve 112 for the blade cylinder 115. The floating operation of the blade cylinder 115 is carried out with the use of an arrangement comprising logic valves 120, 121. The logic valve 120 is disposed in conduit 118 which extends from an "up" port 116 of selector valve 112 to a first port of blade cylinder 115, while the logic valve 121 is disposed in conduit 119 which extends from a "down" port 117 of selector valve 112 to the second port of blade cylinder 115. Each of logic valves 120, 121 is connected to a reservoir tank 127 by way of a return conduit 126 through the intermediary of a solenoid selector valve 123. A solenoid relief valve 124 is connected between the conduit 119 and the return conduit 126. In this hydraulic circuit, the solenoid selector valve 123 is operated by depressing a floating switch (not shown), which is incorporated in the manual control lever 125, so as to simultaneously open the logic valves 120, 121, and thereby connect both of the conduits 118, 119 to the reservoir tank 127 by way of the return conduit 126 so as to allow the blade to be in a condition of floating operation. At the same time, the solenoid relief valve 124 avoids any excessive pressure in the blade cylinder 115 which would be caused by a force from the ground.
However, with the manually controlled circuit shown in FIG. 5, the control of the vertical position of the blade is inferior, and the control of the operation speed is difficult. Further, in a large size construction machine, the size of the selector valve 101 becomes larger as the necessary flow rate of hydraulic fluid is higher, and accordingly, either the control force has to be larger or the stroke of the control lever 102 has to be longer. Accordingly, a problem arises in that such operation tires the operator, thereby lowering the workability of the system.
The hydraulic circuit illustrated in FIG. 6, which has been proposed to eliminate the above-mentioned problem with the system of FIG. 5, requires the presence of the logic valves 120, 121 and the solenoid selector valve 123 in the conduits 118, 119 extending from the "up" and "down" ports 116, 117 to the blade cylinder 115 in order to obtain a "floating" position, and accordingly, the structure becomes large and complicated. In particular, in a large size construction machine, the logic valves 120, 121 and the solenoid selector valve 123 become large, and accordingly, the solenoid assemblies for operating these valves become larger.
Further, in order to set the blade at a "floating" position in a system where the control lever 125 is set at the "floating" position with the use of a detent, which is provided for the selector valve 112 similar to the detent 104 in the manually controlled circuit of FIG. 5, the selector valve 112 is moved to the "floating" position. However, if the control lever 125 is released to be free, the control lever 125 moves to a "hold" position under the reaction force of a spring 114a in the pressure proportional control valve 114. Accordingly, a problem arises in that the operator has difficulty recognizing, from the position of the control lever 125, a condition in which the blade is actually floating.